Hi Tom,
Your webserver is using the response header of
Content-Type:"text/html; charset=UTF-8"
I think you're running a V2 version of the WD-AJAX-PHP template set. The V3 version has a top.php that fixes that issue.
Time to update to V3? See
here for instructions. Since I discontinued the V2 templates in 2011, you've been fairly lucky not having major issues since then, but a lot has changed and upgrading to V3 puts you back in full-support mode